1. Field of the Invention
The present invention relates to an antenna device and a radar apparatus which can detect a target using a reflected wave from a target (object) in response to radiated radio waves of 1 to 10 millimeters. Particularly, the present invention relates to an antenna device mounted on a DBF (Digital Beam Forming) radar apparatus.
2. Background Art
In order to guarantee driving safety of a vehicle, forward-looking on-board radar apparatuses detecting targets such as obstacles in the traveling direction (on the front side) or preceding vehicles have been developed. An on-board radar apparatus including plural receiving antennas and employing a process (DBF method) of phase-shifting and combining received signals from the receiving antennas is known. In order for the on-board radar apparatus to detect a target with high precision, it is desirable to reduce the beam width of a radio wave radiated when carrying out the DBF method and to thin the receiving characteristics of a reflected wave.
In general, in directional characteristics of an antenna in response to signals obtained by performing the DBF method, a component from the direction of a target plays a main role. The directional characteristic in this direction is referred to as a main beam. The directional characteristics of the signals radiated in the other directions are referred to as side lobes. The side lobes are not perfectly zero. In general, it is known that the side lobe can be reduced by increasing the number of elements in an antenna and unevenly arranging the elements.
For example, in the antenna device described in Japanese Patent No. 3283589 (Patent Document 1), an array including plural elements is partitioned into plural sub-arrays and the sub-arrays are formed in a stepped shape in the Y axis direction in which they are uneven in one direction, for example, in the X axis direction.